This invention relates to a curable silicone release coating, to a process for applying such a coating to an oriented polymer film and, in particular, to a coextruded, pressure sensitive, oriented polymeric label stock product possessing an integral release layer obtained by the process.
Silicone compositions have long been used as release coatings for rendering a surface or material relatively nonadherent to other materials which would normally adhere thereto. For example, silicone release compositions have found application as coatings which release pressure-sensitive adhesives for labels, decorative laminates, transfer tapes, etc. Silicone release coatings on paper, polyethylene, polypropylene and many other kinds of films are also useful for providing nonadherent surfaces for food handling and industrial packaging.
One method of producing silicone-coated release papers involves first forming a curable release composition consisting of (1) a hydroxy end-blocked dimethylsiloxane gum stock; (2) a crosslinking agent such as a siloxane fluid containing silicon-bonded hydrogen units (Si--H), (3) a catalyst such as a metal salt of a carboxylic acid and (4) an organic solvent such as xylene. The composition is then applied to the paper substrate in the form of a coating and the coated substrate is heated mildly to drive off the solvent. After the solvent has been removed the siloxane composition on the substrate is cured at relatively high temperatures, e.g., 100.degree. C. to 150.degree. C., to crosslink the siloxane gum and form the desired release paper product.
In order to dispense with the need for a solvent, in recent years a variety of solventless curable silicone release compositions have been developed which are readily curable at relatively low temperature, by light radiation or by some other form of radiation. Illustrative of such silicone release compositions are those described in U.S. Pat. Nos. 3,726,710; 4,070,526; 4,201,808; 4,256,870; 4,262,107; 4,279,717; 4,340,647; 4,347,646; 4,348,454; and 4,576,999, the disclosures of which are incorporated by reference herein.
In the conventional manufacture of pressure sensitive adhesive labels, a solution of a pressure sensitive material is applied to a substrate, or face stock, usually paper, in the form of a continuous web and dried. Thereafter a release sheet, usually provided as a continuous paper web coated with a release agent as described above, is applied to the exposed pressure sensitive adhesive surface to form a substrate-pressure sensitive adhesive-release sheet label stock which is then wound on a rewind roll for further processing such as sheeting, slitting or other conversion. After that, the label stock is typically printed, cut and collated by a printer to form individual printed sheets. Such sheets may be utilized for any number of uses including name tags, stickers, labels, etc., by simply peeling away the release paper and pressing the adhesive-coated side of the face sheet to the desired surface.
U.S. Pat. No. 4,582,736 and 4,626,460 the disclosures of which are incorporated by reference herein, each describes a biaxially oriented coextruded label stock material which comprises: (a) a facing layer in the form of a polyolefin film having an upper surface and a lower surface, and preferably one containing a quantity of opacity-producing voids in accordance with the teachings of U.S. Pat. No. 4,377,616, a pressure sensitive adhesive component incorporated within said facing layer or applied as a coating to the lower surface thereof; and (b) a release layer in the form of a polyolefin film having an upper surface and a lower surface and having a release agent component for the pressure sensitive adhesive component of facing layer (a) incorporated therein or applied as a coating to the upper surface thereof, the lower surface of facing layer (a) being peelably affixed to the upper surface of said release layer (b). The release agent can be incorporated into the polymer constituting release layer (b) prior to extrusion as it will tend to concentrate at the surfaces of this layer following orientation. Alternatively, the release agent can be applied to the surface of release layer (b) in a separate operation. Release agents mentioned in U.S. Pat. No. 4,582,736 and U.S. patent application Ser. No. 814,693 include silicones and modified silicones, the modification including both copolymerization of silicones with other non-release chemical agents or by adding non-silicone materials to a silicone coating solution prior to surface application of the release agent to the release layer. Other release agents indicated as useful include polyethylene, fluorocarbon, the Werner type chromium complexes and polyvinyl octadecyl carbamate.